Hydraulic Actuators

DR. SHANTIPAL S. OHOL,

Associate Professor,
Mechanical Engineering Department,
In-Charge Centralized Robotics & Automation Lab,
Faculty coordinator Robot Study Circle,
College of Engineering , Pune – 411 005.
 Type of Drives
Hydraulic • Precise • More Space
• High Payload • Slow speed
• Noiseless • Leakage
Pneumatic • Economical • Jerky Motions
• Faster Movements • Noise
• Lesser space • Less Accurate
Electric
• Better Control • Less payload
• Compact • Spark
• Easy operations
Hydraulic Actuators
The hydraulic actuator consists of cylinder or fluid motor that uses hydraulic power to
facilitate mechanical operation. The mechanical motion gives an output in terms of linear,
rotatory or oscillatory motion. As liquids are nearly impossible to compress, a hydraulic
actuator can exert a large force. The drawback of this approach is its limited acceleration.

The hydraulic cylinder consists of a hollow cylindrical tube along which a piston can slide.
The term single acting is used when the fluid pressure is applied to just one side of the
piston. The piston can move in only one direction, a spring being frequently used to give
the piston a return stroke. The term double acting is used when pressure is applied on
each side of the piston; any difference in force between the two sides of the piston moves
the piston to one side or the other.
Pneumatic Power

Uses compressible fluid

Parts
◦ compressor,
◦ storage tank
◦ motor or engine

Types:
◦ Single-action
◦ Double action
Single-Action Cylinder

Outward Stroke Return Stroke

Single-Action Cylinder

Outward Stroke
◦ F = (0.7854 x D2 x P) – (S + Ff)
Return Stroke
◦ F = S – Ff
Where
◦ D: diameter of the piston
◦ P: pressure of the fluid entering the
cylinder
◦ S: return spring pressure
◦ Ff: friction force of the piston
Double-Action Cylinder
(a) Outward stroke
(b) Inward Stroke

Hold Position
Double-Action Cylinder

Outward Stroke
◦ F = 0.7854 x D2 x P – Ff
Inward Stroke
◦ F = 0.7854 x (D2 – Dr2) x P – Ff
◦ Where
◦ D: diameter of the piston
◦ P: pressure of the fluid entering the cylinder
◦ Dr: diameter of piston rod
◦ Ff: friction force of the piston
Hydraulic Power
Hydraulic power uses a non-
compressible fluid to transmit energy.
Single-Action Rotary Actuator
T = (P x A x Rc) - Tf
Where
◦ T: torque developed by the actuator
◦ P: fluid pressure
◦ A: vane area
◦ Rc: center radius of the vane,
◦ Tf: friction torque.
Single- and Double- Action Rotary Actuator

T = (2 x P x A x Rc) – Tf
Where
◦ T: torque developed by the
actuator
◦ P: fluid pressure
◦ A: vane area
◦ Rc: center radius of the vane,
◦ Tf: friction torque.
Sample Hydraulic Motors
 Actuators
An actuator is an electromechanical device which converts energy into mechanical work (or
motion). For robots, actuators are like muscles that perform work. The work can be either to induce
motion, or to object motion; i.e. either to start a movement, or to stop it. There are different types of
actuators available and most of them either create rotational motion, or linear motion. (Oscillatory
motion is rarely used, and even if required can be created using a linear or a rotary actuator)

Linear Actuators
As the name says, linear actuator creates linear motion, i.e. it creates to and fro motion. These
actuators can be driven by either linear or rotational motion.
To simplify things, let us take an example of a bicycle. When the cyclist pushes the pedal, it rotates
the bottom bracket in a cycle which is connected to a roller chain. Now the rotational motion from the
bracket creates a linear motion in the roller chain.
 Rotational Actuator (Rotary Actuator)
Rotational actuator induces rotary or rotational motion. A simple DC motor is an example of rotational actuator.
Similar to linear actuator, rotation actuators can be driven by either linear motion or rotational motion.
Let us continue with our bicycle example to understand how linear motion can be used to create rotary motion.
The roller chain in a bicycle is connected to sprocket gear of the driving wheel (Normally back wheel). When a
cyclist pedals, the roller chain (remember that roller chain is an example of linear motion) rotates the sprocket
gear creating a rotational motion further rotates the wheel.
Most actuators can be mechanically designed to induce rotary motion or linear motion. A simple nut attached to a
linear member can create a rotary motion. On the other hand, attaching a screw to a rotary actuator creates linear
motion.
The below animations shows both linear motion and rotary motion. The bottom bracket rotates creating a linear
motion in roller chain. Further the same linear motion of the chain creates rotary motion in sprocket gear.

Actuators require energy to create motion and the source of energy is usually electric current, pneumatic pressure
or hydraulic fluid. In the next section, we will understand the different actuators available for robots, and their
energy sources.
Linear Actuator
Linear actuators can be divided into three types: screw, belt, and rod type.
Lead screw actuators have a threaded nut which moves with respect to the screw. This generates
motion in whichever element is not fixed. This technology is simple, economical, and widely used. All
the screws in screw type actuators are made of lead, but term 'lead scrw actuator' is commonly used
for this design. The disadvantages of this design include the amount of wear that occurs between the
surfaces of the nut and the threads of the screw, which reduces lifetime, efficiency, speed, and
performance. Lead screw actuators are best used when performance trade-offs are acceptable and
when the application requires a lighter load and duty cycle.
Ball screws are lead screw and ball nut combinations that enable the balls in the nut to circulate
when the actuator is in motion. The motion of the nut around the screw is assisted by the ball
bearings. This reduces friction, distributes the load, and increases the lifetime predictability over a
lead screw design. The advantages of ball screws include the ability to take heavy loads, deliver
precision positioning, and higher efficiency and thrust capabilities than a lead screw actuator. The
disadvantages associated with ball screws is that they are more expensive, generate more noise, and
the bearings can become contaminated reducing performance or causing failure.

Planetary roller screw uses a planetary arrangement of threaded rollers surrounding the main
threaded shaft. This increases the surface area that takes the load and offers the highest possible
thrust and lifetime of the screw type actuators. Planetary roller screws are the most expensive type of
screw actuator but they have the best performance for demanding applications that require high thrust
force.
Linear Actuator Specifications
Linear actuators vary in terms of motor type, power, and features. A linear motor is very
similar to a rotary electric motor. In a linear motor the rotor and stator (metal ring of insulated
wire) components are laid out in a straight line. The magnetic field structures of linear motors
are physically repeated across the length of the actuator.

Linear motors generate force only in the direction of travel, and do not utilize a rotary
mechanism to transfer power.
Motor specifications
•Motor voltage denotes the voltage applied to the motor and includes AC, DC, and stepper type
motors.
•Continuous power, also known as sustainable power, does not include short-term peak power
ratings.
Motor features
•Motor encoder feedback provides continuous output position in an analog or digital signal.
•Linear position feedback provides continuous output of position in an analog or digital signal.
•Position switches have a switch that outputs the limit travel.
•Integral brakes hold the current position of the motor.
Actuator Features
Linear actuators provide many optional features.
•Adjustable stroke- Adjustable stroke allows for the end points or the total stroke length to be adjusted.
•Bumpers and cushions-Bumpers and cushions are used to soften the impact at the end of the stroke.
•Closed loop control- Also known as servo control, the cylinders have external devices that send back a signal to the pump
control giving it position information.
•Holding brakes- Holding brakes work in conjunction with the self-locking feature to increase holding force.
•Shock absorbers- Shock absorbers are used in pneumatic of hydraulic fluid absorption of shock.
•Double-ended rods- Double end rods extend from both ends of the cylinder with attachment features such as threads on both ends.
•Multi-position end-plate- A multi-position end plate can be actuated to different positions along its stroke, not just the endpoint.
•Integrated overload slip clutch or torque limiter- An integral flow control incorporates a flow control valve that limits the
amount of air or fluid that enters the cylinder.
•Protective boot- Protective boots are a cover that protects moving parts against environment damage.
•Self-locking- Self-locking actuators lock in the current position when there is a loss of signal.
•Integral sensors- Integral sensors are equipped within the cylinder to monitor position and proximity.
•Integral flow- Integral flow control incorporates a flow control valve or device that limits the amount of air of fluid that
enters the cylinder.
•Non-rotating- Non-rotating denotes multiple rods to prevent the plunger from rotating.
•Magnetic switches- Magnetic switches such as Hall Effect sensors indicate if the thruster is in the retracted or extended position.
•Thermal overload protection- Thermal overload protection is used to trip a switch when a preset temperature is exceeded.
•Intrinsically safe- Intrinsically safe electric linear actuators can be used in hazardous environments e.g. chemical processing facilities.
•Water resistant- Devices are sealed to prevent corrosion due to water or liquid entering the housing.
DESIGN CRITERIA IN HYDRAULIC CYLINDERS. ...

•Temperature: Media and working condition temperature should be considered when choosing suitable
sealing element.

•Pressure: System pressure and pressure type should be considered when choosing suitable sealing element.

•Sliding Speed: ...

•Media: ...

•Working Conditions.
Courtesy :- https://www.valin.com
Courtesy :- https://www.valin.com
a load of 1000 N required to be pushed by a hydraulic cylinder and the fluid is
pumped at a pressure of 2 N/mm2. Find out the required piston size.

Solution:
Step 1: First find the area of cross section required:
A = F/ P
= 1000/2
= 500 mm2
Step 2: Calculate the piston diameter
Dp = √(4A/π)
= √(4*500/3.14)
= 25.23mm
1. A Cylinder has a bore of 80mm diameter and a rod of 45mm diameter. It drives a load of 7000N, travelling at a
velocity of 15m/min. The load slides on a flat horizontal surface having a coefficient of friction of 0.12. The load is to
be decelerated to rest within a cushion length of 20mm. If the relief valve is set at 50 bar, compute the fluid pressure
developed in the cushion.
Cushion length s = 20mm = 0.02m
Velocity u = 15 m/min = 0.25 m/s
From the equation of motion,
v2 = u2 + 2as (final velocity is zero)
a = - u2 / 2S
Decelerating force to retard load = (w / g) X (a) = (w / g) x (- u2 / 2S)
= 6700 x (0.25)2 / (9.81 x 0.02 x 2) = 1067 N

Pressure force on blank end = P x A = (50 x 105) x (π / 4) x (0.08)2 = 25133N

Friction force = μ.w
= 0.12 x 6700 = 804 N
Cushion force = (Pressure force + Decelerating force ) – Friction force
= 25133 + 1067 – 804
= 25396N
Fluid pressure developed at the cushion = F / AP - AR
= 25396 / ((π / 4) x (0.082 – 0.0452 )
= 74 bar
 3. A hydraulic cylinder has to move a table of weight 13kN. Speed of the cylinder
is to be accelerated up to a velocity of 0.13m/s in 0.5 seconds and brought to stop
within a distance of 0.02m. Assume coefficient of sliding friction as 0.15 and
cylinder bore diameter as 50mm. Calculate the surge pressure.